Many 3-substituted-4-oxo-4H-benzopyrans are known and various 3-phenyl derivatives (isoflavones) have been proposed for use as feed additives for promoting the growth or improving feed conversion efficiency in econically important animals. Other 3-substituted-4-oxo-4H-benzopyrans have been proposed as anti-allergic drugs.
Existing processes for the preparation of such compounds are frequently multistage and of limited utility, as for example, the synthesis of isoflavones described by Baker and Ollis, Nature 169, 706 (1952) and by Baker, Chadderton Harborne and Ollis, J. Chem. Soc., 1852 (1953). The use of the Vilsmeier reaction has been described by Kagal, Nair and Venkataraman, Tetrahedron Letters 593 (1962) but this process is not of general utility. For example, it fails in the case of polyhydroxy-phenyl ketones. In addition, when isoflavones are prepared by this process, the major product is commonly a compound in which the aromatic ring is formylated.